Petroleum and natural gas producing wells typically employ a plunger disposed within tubing of the well. The plunger provides lift to liquids accumulated above the plunger in the wellbore, powered by gas and pressures below the plunger from formations in the earth which are in communication with the lower part of the well, below the plunger, relying on variable fluid pressures within the well-bore, above and below the plunger. The well-bore is typically lined with tubular materials of relatively uniform internal surface diameter, but operators expect the internal passageway of the tubular to be somewhat uneven or imperfect. It is optimal if the gap between the outer sides of the plunger and the inner surface of the tubular is kept small, as this will make the lift system operate more efficiently, as less pressure and fluid from beneath will bypass the plunger, and less fluid above the plunger can drop below, past the plunger. In essence, it would be ideal to have a plunger which was perfectly sealed to the tubular but moved frictionlessly along its length in either direction, powered by fluid pressure variations above and below the plunger (at least on the up-stroke lift portion of the plunger system's cycle). It is also useful to have replaceable surfaces on the outer sides of the plunger as that surface will wear from contact with the tubular's inner wall; an outer surface of different materials from the plunger's body may also be advantageous as different materials can be used to provide different structural, mass and density, permeability, chemical reactivity, formability or machineability, resilience, tooling, frictional, or wear or other characteristics as required for manufacturing, operation, assembly, repair, or function in place of different parts of the plunger.
In the prior art, a variety of mechanical plungers for use in gas-lift systems for production of fluids from wells have been disclosed or are known. Each has disadvantages. Some examples follow:
U.S. Pat. No. 6,725,916 to Gray et al. (“Gray”) discloses a plunger with a system of floating, spring-loaded pads between a plunger's body and the tubular within which it operates, together with a novel seal and internal passage, with the aim of facilitating rapid descent of the plunger from its upper-most part of a stroke in its lift-cycle (by opening the inner passage at the top of the stroke, and reclosing it at the bottom). Gray provides a good example of state-of-the art pad systems. Gray's “jacket” comprises a series of interlocking pads held to the plunger's body but spring-loaded to bias outwardly toward the tubular's walls. The aspect in Gray's jacket which is relevant, is the provision of “labyrinthine passages” between the jacket's elements (the spaces between the pads), which in Gray are formed by the interlocking teeth of each pad with the adjacent pads—when the jacket (pads) is expanded, the spaces between the interlocking pads increases, providing larger and larger flow-paths for fluid communication past the plunger in the tubular during use. This is undesirable, and Gray has attempted to resolve the issue by making these passages between the interlocking pads “labyrinthine” or following a toothed, notched, or circuitous pathway. Notably, the notch-finger interlocking region between pads in Gray are also stepped and matched with a step in the extended end of each finger (and a mating void in the recess or notch into which the finger fits when assembled) which is stepped up and down in a direction radial to the linear centre of the plunger (to its longitudinal axis), while the rest of the adjacent pads' mating surface edges are not stepped in that way. While providing some resistance to fluid flow past the plunger in the annulus between the plunger and the tubular, there is still a void and passageway for fluid communication with a large cross-section.
US Patent Application 2012/0080196 by Laing (“Laing”) discloses a plunger lift and safety valve system with a variable outside diameter plunger where the diameter is variable by the retraction and expansion of pads deployed about the outer circumference of the plunger's body between the plunger and the tubular (when expanded) or between the plunger and a smaller-diameter (than the tubular) safety valve (when the pads are retracted). The pads are spring-biased toward the inner surface of the tubular from the plunger, and are interlocking with each other to permit them to radially expand and contract but to be firmly held linearly in position with the plunger (linearly along the direction of the plunger's longitudinal axis). The relevance of Laing as an example of prior art plunger pad systems is that the pads interlock and are biased outwardly by springs, but when expanded the spaces between the pads open up, providing a large cross-section (viewed longitudinally along the plunger's axis to a cross-section of the plunger and pads), the openings between the pads are the relevant flow-paths for fluid flow past the plunger, which is undesirable both in terms of efficiency of operation, as well as contamination of the plunger's working parts with materials produced with the hydrocarbon fluids in the well (debris, sand, silt, corrosive materials, etc).
It is an object of the present invention to obviate or mitigate at least one disadvantage of previous related art.